I want to make my app without a header at the top. How do I do this? Is doing it full screen enough?

You don't have to have a header but there is a difference between that and the full-screen mode. In the regular mode (with or without header), when you get a dashboard alert, your display area becomes smaller and your app card will shrink.

In the full-screen mode, this does not happen, but you will also not see the alert unless you minimize the app card.